Temperature compensation for a voltage controlled oscillator

ABSTRACT

An apparatus that is comprised of a controller, a digital-to-analog converter (DAC), a temperature sensor, an analog-to-digital converter (ADC), and a voltage controlled oscillator (VCO). The controller to reads temperature data proportional to a temperature of the VCO, reads previously-calculated calibration data based on the read temperature data, determines a frequency command signal based on the read previously-calculated calibration data, and outputs the frequency command signal. The DAC converts the frequency command signal into a frequency analog signal. The temperature sensor produces the temperature signal. The ADC converts the temperature signal into the temperature data. The VCO produces an output frequency based on the frequency analog signal.

CROSS-REFERENCE TO RELATED APPLICATION

NA

BACKGROUND OF THE DISCLOSURE 1. Field of the Disclosure

The disclosure relates in general to temperature compensation, and moreparticularly, to temperature compensation for a voltage controlledoscillator.

2. Background Art

In typical RF signal generation, accurate and repeatable frequencyoutputs are generated utilizing a voltage controlled oscillator (VCO).The VCO is an electrical component that, given a specific input voltage,instantaneously outputs a specific frequency. Many commercialoff-the-shelf VCOs are sensitive to temperature changes such that, givena constant voltage input, their outputs will drift as the temperaturechanges, thereby introducing inaccuracy in the expected outputfrequency. Further, VCOs tend to be nonlinear devices and requirelinearization for accurate frequency generation. Lastly, behavior ofVCOs can vary from device to device, even if they are the samemanufactured part.

SUMMARY OF THE DISCLOSURE

The disclosure is directed to an apparatus that is comprised of acontroller, a digital-to-analog converter (DAC), a temperature sensor,an analog-to-digital converter (ADC), and a voltage controlledoscillator (VCO). The controller reads temperature data proportional toa temperature of the VCO, reads previously-calculated calibration databased on the read temperature data, determines a frequency commandsignal based on the read previously-calculated calibration data, andoutputs the frequency command signal. The DAC converts the frequencycommand signal into a frequency analog signal. The temperature sensorproduces the temperature signal. The ADC converts the temperature signalinto the temperature data. The VCO produces an output frequency based onthe frequency analog signal.

In some configurations, a scaling amplifier is coupled to an output ofthe DAC and an input of the VCO. The scaling amplifier adjusts thefrequency analog signal to an appropriate voltage level for the VCO.

In some configurations, a memory is coupled to the controller, thememory storing the previously-calculated calibration data.

In some configurations, the controller is a micro-controller.

In some configurations, a power amplifier is coupled to an output of theVCO, the power amplifier amplifying the output of the VCO.

In some configurations, the controller further interpolates between twopreviously-calculated calibration data to determine the frequencycommand signal.

In some configurations, the previously-calculated calibration data iscomprised of N+1 coefficients for an N-order polynomial that maps theoutput frequency of the VCO to an input voltage of the VCO.

In some configurations, the N-order polynomial is:V=(c ₀ ×F ^(N))+(c ₁ ×F ^(N-1))+(c ₂ ×F ^(N-2))+ . . . +(c _(N) ×F ⁰);

where c_(i) is the i^(th) coefficient for a calibration point, F is theoutput frequency of the VCO, N is the order of the polynomial, and V isan input voltage of the VCO that results in the output frequency F ofthe VCO.

In some configurations, the previously-calculated calibration data isderived from a one-time calibration process that takes place aftermanufacture of the apparatus, with a temperature controlled environmentbeing used to bring the apparatus through various operating temperaturesof the apparatus to produce the previously-calculated calibration data.

The disclosure is also directed to a method comprising readingpreviously-calculated calibration data from a memory and outputting,from a controller, a frequency command signal based on thepreviously-calculated calibration data. The method further comprisesconverting the frequency command signal into a frequency analog signaland applying the frequency analog signal to a voltage controlledoscillator (VCO) to produce an output frequency. The method furthercomprises determining if the temperature of the VCO has changed beyond athreshold value and, subsequent to the determining, reading temperaturedata representing a current temperature of the VCO. The method furthercomprises, if the temperature of the VCO has changed beyond thethreshold value, branching to the reading the previously-calculatedcalibration data and, if the temperature of the VCO has not changedbeyond the threshold value, branching to the reading temperature data.

In some configurations, the reading of the temperature data is a secondreading of the temperature data, the method further comprises, prior tothe reading the previously-calculated calibration data from the memory,first reading temperature data representing the current temperature ofthe VCO.

In some configurations, the method further comprises adjusts thefrequency analog signal to an appropriate voltage level for the VCO.

In some configurations, the previously-calculated calibration data ofthe method includes N+1 coefficients for an N-order polynomial that mapsthe output frequency of the VCO to an input voltage of the VCO.

In some configurations, the N-order polynomial of the method is:V=(c ₀ ×F ^(N))+(c ₁ ×F ^(N-1))+(c ₂ ×F ^(N-2))+ . . . +(c _(N) ×F ⁰);

where c_(i) is the i^(th) coefficient for a calibration point, F is theoutput frequency of the VCO, and V is an input voltage of the VCO thatresults in the output frequency F of the VCO 110.

In some configurations, the method further comprises power amplifying anoutput of the VCO.

In some configurations, the method further comprises deriving thepreviously-calculated calibration data from a one-time calibrationprocess by bringing the VCO through various operating temperatures toproduce the previously-calculated calibration data.

In some configurations, the method further interpolates between twopreviously-calculated calibration data to determine the frequencycommand signal.

In some configurations, an apparatus executes the method.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will now be described with reference to the drawingswherein:

FIG. 1 illustrates an apparatus for compensating for temperature changesof a VCO, in accordance with the embodiments disclosed herein;

FIG. 2 illustrates a flowchart for an example method of initiallycalibrating the apparatus of FIG. 1, in accordance with the embodimentsdisclosed herein; and

FIG. 3 illustrates a flowchart for an example method of operation of theapparatus of FIG. 1, in accordance with the embodiments disclosedherein.

DETAILED DESCRIPTION OF THE DISCLOSURE

While this disclosure is susceptible of embodiment in many differentforms, there is shown in the drawings and described herein in detail aspecific embodiment(s) with the understanding that the presentdisclosure is to be considered as an exemplification and is not intendedto be limited to the embodiment(s) illustrated.

It will be understood that like or analogous elements and/or components,referred to herein, may be identified throughout the drawings by likereference characters. In addition, it will be understood that thedrawings are merely schematic representations of the embodiments, andsome of the components may have been distorted from actual scale forpurposes of pictorial clarity.

The embodiments described herein provide both an apparatus 100 andmethod 300 that compensate for inaccuracies of a typical VCO, such as insystems that use a VCO in an “open-loop” configuration. VCOs are used inopen-loop configurations in many RF-based applications. There arereasons why a system designer might choose to use a VCO in an open-loopconfiguration, such as a) size constraints: extra circuitry required forusing a phase-locked loop (PLL), b) speed: changing VCO frequencies cantake a while for the PLL to “lock” onto the target frequency, c)complexity: designing associated PLL circuitry and programming the PLLcan be complicated. The apparatus 100 uses curve fitting to modelbehavior of a VCO 110 at specific temperatures and then subsequentlyuses resulting polynomial coefficients to determine, in real time,appropriate input voltages for the VCO 110 for any desired valid outputfrequency, without being susceptible to temperature-induced frequencydrift.

Referring now to the drawings and in particular to FIG. 1, an apparatus100 is disclosed for compensating for temperature changes of a VCO 110,in accordance with the embodiments disclosed herein. The apparatus 100includes a controller 102, such as micro-controller, coupled to anoscillator module 108. The controller 102 includes a digital-to-analogconverter (DAC) 104. The oscillator module 108 includes the VCO 110 anda temperature sensor 112 which are tightly coupled together. In anembodiment, the controller 102 is coupled to the VCO 110 via a scalingamplifier 106, with the scaling amplifier 106 coupled to an output ofthe DAC 104 and an input of the VCO 110. The temperature sensor 112includes an analog-to-digital converter (ADC) 114. In anotherembodiment, the ADC 114 is a discrete component from the temperaturesensor 112. The controller 102 is further coupled to a memory 118.

During operation of the apparatus 100, the temperature sensor 112 sensesthe temperature of the VCO 110 and produces a temperature signal that isproportional to the sensed temperature of the VCO 110. The ADC 114converts this temperature signal into the temperature data (digitaldata), the temperature data thus representing the temperature of the VCO110. The memory 118 stores previously-calculated calibration data. Thecontroller 102 reads the temperature data and reads thepreviously-calculated calibration data from the memory 118 based on theread temperature data. The controller 102 further outputs a frequencycommand signal (digital data) based on the read previously-calculatedcalibration data. The DAC 104 converts this frequency command signalinto a frequency analog signal. The VCO 110 operates within a specificrange of input voltage levels, by design. In an embodiment, the scalingamplifier 106 adjusts the output of the DAC 104 to an appropriatevoltage level within this range of input voltage levels, that isappropriate for the VCO 110.

The VCO 110 produces a desired output frequency based on the adjustedversion of the frequency analog signal produced by the scaling amplifier106. Thereafter, the power amplifier 116 amplifies the output frequencyto a desired level, the desired level being application specific.

The DAC 104 converts a frequency command signal produced by thecontroller 102 into a frequency analog signal. In another embodiment,the DAC 104 is a discrete component from the controller 102. The DAC 104is coupled to a scaling amplifier 106. The scaling amplifier 106 adjuststhe frequency analog signal to an appropriate voltage level for input tothe VCO 110, with the VCO 110 receiving the adjusted signal produced bythe scaling amplifier 106.

The apparatus 100 solves the problem of temperature-induced outputfrequency drift for the VCO 110 by generating a series oftemperature-specific models such that, given a desired output frequencyof the VCO 110 for a specific temperature, the models will generate theappropriate input voltage of the VCO 110. During runtime, the controller102 can monitor the temperature of the VCO 110, e.g. via the temperaturesensor 112. If this temperature has deviated beyond some specificapplication-defined threshold, then the appropriate model is read by thecontroller 102 from the memory 118 and applied to compensate for theoutput frequency drift of the VCO 110.

As an example, the VCO 110 has an input range of 0V to 3V and an outputrange of 2.25 GHz to 2.5 GHz at 25° C. Ideally, the relationship betweenthe input of the VCO 110 and the output of the VCO 110 is perfectlylinear. That is, an input of 0V would result in an output of 2.25 GHzand an input of 3V would result in an output of 2.5 GHz. Given thisrelationship, a general formula calculates the necessary input voltagefor a given desired output frequency:

$\begin{matrix}{V_{n} = {V_{\min} + ( {\frac{F_{n} - F_{\min}}{F_{\max} - F_{\min}} \times ( {V_{\max} - V_{\min}} )} )}} & ( {{Eq}.\mspace{14mu} 1} )\end{matrix}$

Where:

V_(min) and V_(max) are the minimum and maximum VCO input voltages (e.g.0 and 3V for our example);

F_(min) and F_(max) are the minimum and maximum VCO output frequencies(e.g. 2.25 GHz and 2.5 GHz for our example);

F_(n) is the desired output frequency; and

V_(n) is the VCO input voltage that should be applied to the VCO thatresults in the output frequency F_(n) of the VCO 110.

Further to this example, there is a desire to output a frequency of2.404 GHz. Using Equation 1 above, an input of 1.848V is need for theVCO 110. However, in practice when an input of 1.848V is applied to theVCO 110, the output of the VCO 110 may be 2.404 GHz while a temperatureof the VCO 110 is approximately 25° C. But, as the VCO 110 heats up (orcools down) that same input voltage results in a drifting outputfrequency. This is a problem for systems that need to generate anaccurate frequency over a range of temperatures.

The apparatus 100 solves this drifting output frequency problem byalternatively determining the appropriate input voltage for a desiredoutput frequency that takes a temperature of the VCO 110 intoconsideration. Instead of using Equation 1, the apparatus 100, andspecifically the controller 102, uses the previously-calculatedcalibration data that allows modeling of behavior of the VCO 110 at aparticular temperature. More specifically, this previously-calculatedcalibration data consists of N+1 coefficients for an N-order polynomialthat maps a desired output frequency of the VCO 110 to an appropriateinput voltage of the VCO 110. The order of the polynomial used isapplication specific and is selected based on characteristics of theparticular VCO 110.

As an example, the VCO 110 has been previously calibrated and 4^(th)order polynomials are used for the apparatus 100. Further, a temperatureof the VCO 110 is 37° C. and has changed enough that compensation forthe temperature change is needed. The controller 102 access the memory118 that stores a lookup table, such as that being previouslyestablished by a one-time calibration process described below in FIG. 2,to read the calibration data for 37° C. The controller 102 reads 5coefficients (1+polynomial order):

515.4323317, −5176.97395, 19486.80889, −32561.62891, and 20371.32458

Given these coefficients, the controller 102 calculates the appropriateinput voltage for the VCO 110 for any valid output frequency (at 37° C.)by evaluating the polynomial:V=(c ₀ ×F ^(N))+(c ₁ ×F ^(N-1))+(c ₂ ×F ^(N-2))+ . . . +(c _(N) ×F⁰)  (Eq. 2)

Where:

c_(i) is the i^(th) coefficient for the calibration point;

F is the desired output frequency in GHz; and

V is the input voltage of the VCO 110 that results in the desired outputfrequency F of the VCO 110.

Further to the example above, to generate an output frequency of 2.404GHz a calculation is performed by the controller 110 as follows:V_(2.404)=(515.4323317×2.404⁴)+(−5176.97395×2.404³)+(19486.80889×2.404²)+(−32561.62891×2.404¹)+(20371.32458×2.404⁰)

With V_(2.404)=1.828.

For the particular VCO 110 of this example at 37° C., if the desiredoutput frequency is 2.404 GHz then the input voltage of 1.828V is to beapplied to the VCO 110. This input voltage of the VCO 110 is somewhatclose to the 1.848V calculated by the idealized linear Equation 1 above,with the disparity being due to a sensitivity of the VCO 110 to highertemperature(s).

To further increase accuracy of the apparatus 100 when a currenttemperature falls between two calibration points, linear interpolationbetween the two calibrated voltages is used. For example, suppose thatthe temperature sensor 112 senses that the VCO 110 is at 37.875° C. andthe desired output frequency of the VCO 110 is 2.404 GHz. From thedescription above, at 37° C. the input of the VCO 110 is 1.828V.Further, using Equation 2 with the calibration data for 38° C. (omittedhere for brevity) the calculated input of the VCO 110 is 1.833V. Thecontroller 102 performs linear interpolation between these two values toobtain the input voltage for 37.875° C.:V _(2.404)=((1−0.875)×1.828)+(0.875×1.833)=1.832

Using this scheme, the apparatus 100 can be as accurate as needed interms of the desired output frequency of the VCO 110 by tuningcalibration resolution (e.g. calibrate every N degrees C.), updatefrequency (e.g. compensate for temperature whenever it's changed Mdegrees C.), and a temperature polling frequency of the VCO 110.

In an example embodiment, the calibration data itself is derived from aone-time calibration process. In an example, this calibration processcan be performed at the factory where the VCO 110 is manufactured.During the calibration process, a temperature controlled environment isused to bring the apparatus 100 through all potential operatingtemperatures. At each calibration temperature as reported by thetemperature sensor 112 of the oscillator module 108, the following stepsare performed:

1. A specific, known voltage is input to the VCO 110;

2. The resulting output frequency is measured using a calibratedspectrum analyzer (not shown);

3. Steps 1 and 2 are performed for 32 different voltages spanning anoperating range of the VCO 110;

4. These 32 voltage/frequency pairs are run through an N-orderpolynomial curve fit algorithm that generates the N+1 coefficients; and

5. The N+1 coefficients (i.e. the calibration data) are stored in thememory 118 such that the controller 102 can read them during normalruntime operation of the apparatus 100.

The amount of storage within the memory 118 required for the calibrationdata is a function of calibration resolution, polynomial order, andunderlying coefficient data type. For example, calibrating the apparatus100 from 15 to 70° C. in one degree increments, using fourth orderpolynomials and 8-byte double-precision floating point coefficients, theamount of storage needed within the memory 118 is((70−15+1)×(4+1)×8)=2,240 bytes. Such small number of bytes for storagewithin the memory 118 allows for use of a small memory 118, whichminimizes the expense of manufacturing the apparatus 100. Further, asthis calibration data is immutable it can be stored in the memory 118,which can be a read-only memory (ROM), such as an Electrically ErasableProgrammable ROM (EEPROM).

FIG. 2 illustrates a flowchart for an example method 200 of initiallycalibrating the apparatus 100 of FIG. 1, in accordance with theembodiments disclosed herein. In an example, the apparatus 100 executesthe method 200. For the method 200, the apparatus 100 is placed within atemperature controlled environment. In block 202, a calibrationtemperature (CalTemp) of the VCO 110 within the temperature controlledenvironment is set at a minimum calibration temperature, such as aminimum potential temperature that the oscillator module 108 is to beoperated within. In block 204, the temperature sensor 112 senses thecurrent temperature of the VCO 110 and the CalTemp from block 202 iscompared, by the controller 102, to the current temperature of the VCO110. If the current temperature of the VCO 110 as sensed by thetemperature sensor 112 is not equal to the CalTemp, block 202 repeats tocontinue to determine if the current temperature of the VCO 110 is equalto the Cal Temp. If the current temperature of the VCO 110 as sensed bythe temperature sensor 112 is equal to the CalTemp, block 204 branchesto block 206.

At block 206, the controller 102 collects N voltage/frequency pairs,such as discussed above. Block 206 proceeds to block 208. At block 208,the controller 102 applies curve fitting to generate the N+1coefficients, as discussed above. Block 208 proceeds to block 212. Atblock 212, the controller 102 stores in the memory 118 the N+1coefficients and applies these coefficients to produce the frequencyanalog signal discussed above. Block 212 proceeds to block 210. At block210, the desired output frequency of the VCO 110 is validated while at acurrent temperature. For example, the calibrated spectrum analyzer canbe used to measure the desired output frequency of the VCO 110 while ata current temperature, as a basis for validating if the coefficientsapplied in block 212 produce the desired output frequency of the VCO 110while at a current temperature. Block 210 proceeds to block 214.

At block 214, the current temperature within the temperature controlledenvironment is incremented. For example, the CalTemp is incrementallyincreased. Block 214 proceeds to block 216. At block 216, the controller102 determines if the incremented CalTemp from block 216 is greater thana maximum calibration temperature, such as a maximum potentialtemperature that the oscillator module 108 is to be operated within. Ifthe controller 102 determines in block 216 that the CalTemp is greaterthan the maximum calibration temperature, the method 200 ends.Otherwise, if the controller 102 determines in block 216 that theCalTemp is not greater than the maximum calibration temperature, block216 branches to block 204 for the controller 102 to continue to collectadditional N voltage/frequency pairs.

FIG. 3 illustrates a flowchart for an example method 300 of operation ofthe apparatus 100 of FIG. 1, in accordance with the embodimentsdisclosed herein. In an example, the apparatus 100 executes the method300. In block 308, the temperature of the VCO 110 is read. For example,the temperature sensor 112 produces a temperature signal that isproportional to a temperature of the VCO 110 and the ADC 114 convertsthis temperature signal into the temperature data. The controller 102reads this temperature data in block 308. Block 308 proceeds to block310.

At block 310, temperature compensation is performed. For example, thecontroller 102 reads the previously-calculated calibration datadiscussed above from the memory 118. The controller 102 determines thefrequency command signal based on the read previously-calculatedcalibration data. The controller 102 outputs the frequency commandsignal. The DAC 104 converts this frequency command signal into afrequency analog signal. The VCO 110 operates within a range of inputvoltage levels by design. In an embodiment, the scaling amplifier 106adjusts the output of the DAC 104 to an appropriate voltage level withinthis range of input voltage levels, that is appropriate for the VCO 110.The scaling amplifier 106 applies the adjusted frequency analog signalto the VCO 110. The VCO 110 produces a desired output frequency based onthe adjusted version of the frequency analog signal produced by thescaling amplifier 106. Branch 310 proceeds to block 312.

At block 312, the CalTemp is set to be equal to the current temperatureof the VCO 110. In an example, the controller 102 sets the CalTemp to beequal to the current temperature of the VCO 110. Block 312 proceeds toblock 302. At block 302, a delay is added to the method 300. In anembodiment, this delay is optional. Block 302 proceeds to block 304. Atblock 304, the temperature sensor 112 senses the current temperature ofthe VCO 110. For example, the temperature sensor 112 produces atemperature signal that is proportional to a temperature of the VCO 110and the ADC 114 converts this temperature signal into the temperaturedata. The controller 102 reads this temperature data in block 304. Block304 proceeds to block 306.

At block 306, a determination is made if the temperature of the VCO 110has changed beyond a threshold value. The temperature of the VCO 100that is read in block 304 is subtracted from the CalTemp from block 312,and an absolute value of this difference is determined. The absolutevalue of the difference is compared to a maximum difference value, MaxDelta. In an example, the controller 102 performs this subtraction,determines the absolute value of this difference, and compares thedifference between the absolute value and the Max Delta. If thiscomparison is greater than the Max Delta, block 306 branches to block310. If this comparison is not greater than the Max Delta, block 306branches to block 302.

The foregoing description merely explains and illustrates the disclosureand the disclosure is not limited thereto except insofar as the appendedclaims are so limited, as those skilled in the art who have thedisclosure before them will be able to make modifications withoutdeparting from the scope of the disclosure.

What is claimed is:
 1. An apparatus, comprising: a controller to readtemperature data proportional to a temperature of a voltage controlledoscillator (VCO), read previously-calculated calibration data based onthe read temperature data, branch to a determination of a frequencycommand signal based on the read previously-calculated calibration dataif the temperature of the VCO has changed beyond a threshold value,branch to the read of the temperature data if the temperature of the VCOhas not changed beyond the threshold value, and output the frequencycommand signal; a digital-to-analog converter (DAC) to convert thefrequency command signal into a frequency analog signal; a temperaturesensor to produce the temperature signal; and an analog-to-digitalconverter (ADC) to convert the temperature signal into the temperaturedata; wherein the VCO produces an output frequency based on thefrequency analog signal.
 2. The apparatus of claim 1, further comprisinga scaling amplifier, coupled to an output of the DAC and an input of theVCO, to adjust the frequency analog signal to an appropriate voltagelevel for the VCO.
 3. The apparatus of claim 1, further comprising amemory coupled to the controller to store the previously-calculatedcalibration data.
 4. The apparatus of claim 1, wherein the controller isa micro-controller.
 5. The apparatus of claim 1, further comprising apower amplifier, coupled to an output of the VCO, to amplify the outputof the VCO.
 6. The apparatus of claim 5, wherein the controller furtherinterpolates between two previously-calculated calibration data todetermine the frequency command signal.
 7. The apparatus of claim 1,wherein the previously-calculated calibration data is comprised of N+1coefficients for an N-order polynomial that maps the output frequency ofthe VCO to an input voltage of the VCO.
 8. The apparatus of claim 7,wherein the N-order polynomial is:V=(c ₀ ×F ^(N))+(c ₁ ×F ^(N-1))+(c ₂ ×F ^(N-2))+ . . . +(c _(N) ×F ⁰);where c_(i) is the i^(th) coefficient for a calibration point, F is theoutput frequency of the VCO, and V is an input voltage of the VCO thatresults in the output frequency F of the VCO.
 9. The apparatus of claim1, wherein the previously-calculated calibration data is derived from aone-time calibration process that takes place after manufacture of theapparatus, with a temperature controlled environment being used to bringthe apparatus through various operating temperatures of the apparatus toproduce the previously-calculated calibration data.
 10. The apparatus ofclaim 1, wherein the DAC further applies the frequency command signaldirectly to the VCO to produce the output frequency.
 11. A method,comprising reading, by a controller, previously-calculated calibrationdata from a memory; reading temperature data representing a temperatureof a voltage controlled oscillator (VCO); determining if the temperatureof the VCO has changed beyond a threshold value; if the temperature ofthe VCO has changed beyond the threshold value, branching to adetermination, by the controller, of a frequency command signal based onthe read previously-calculated calibration data; outputting, by thecontroller, the frequency command signal; converting the frequencycommand signal into a frequency analog signal; applying the frequencyanalog signal to the VCO to produce an output frequency; determining ifthe temperature of the VCO has not changed beyond a threshold value; ifthe temperature of the VCO has not changed beyond the threshold value,branching to the reading of the temperature data; and if the temperatureof the VCO has changed beyond the threshold value, branching to thereading the previously-calculated calibration data.
 12. The method ofclaim 11, wherein the reading the temperature data is a second read ofthe temperature data, the method further comprising, prior to thereading the previously-calculated calibration data from the memory,first reading temperature data representing a current temperature of theVCO.
 13. The method of claim 11, further comprising adjusting thefrequency analog signal to an appropriate voltage level for the VCO. 14.The method of claim 11, wherein the previously-calculated calibrationdata includes N+1 coefficients for an N-order polynomial that maps theoutput frequency of the VCO to an input voltage of the VCO.
 15. Themethod of claim 14, wherein the N-order polynomial is:V=(c ₀ ×F ^(N))+(c ₁ ×F ^(N-1))+(c ₂ ×F ^(N-2))+ . . . +(c _(N) ×F ⁰);where c_(i) is the i^(th) coefficient for a calibration point, F is theoutput frequency of the VCO, and V is an input voltage of the VCO thatresults in the output frequency F of the VCO.
 16. The method of claim11, further comprising power amplifying an output of the VCO.
 17. Themethod of claim 11, further comprising adding a delay prior to thesecond reading the temperature data.
 18. The method of claim 11, furthercomprising deriving the previously-calculated calibration data from aone-time calibration process by bringing the VCO through variousoperating temperatures to produce the previously-calculated calibrationdata.
 19. The method of claim 11, further comprising interpolatingbetween two previously-calculated calibration data to determine thefrequency command signal.
 20. An apparatus to execute the method ofclaim
 11. 21. The method of claim 11, wherein the frequency commandsignal is directly applied to the VCO to produce the output frequency.